The invention relates to hydrogen storage materials. In particular, the invention relates to reversible hydrogen storage compositions.
Hydrogen is a candidate for the next generation of energy carriers, which are needed to meet the challenges of global warming and finite fossil fuel-based energy resources. Over the years, considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are being rapidly depleted, the supply of hydrogen is not so restricted. Hydrogen can be produced from coal, natural gas and other hydrocarbons, or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels by methods such as the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of most, if not all, chemical fuels. Application of hydrogen as a fuel is attractive because it generates no polluting emissions since the main by-product of burning hydrogen is water. However, the use of hydrogen as a source of energy has been hindered due to volumetric problems of storing hydrogen in gaseous or even liquid forms.
Hydrogen storage alloys have been proposed and developed to the extent of commercial use in metal hydride batteries. However, the gravimetric hydrogen storage in alloys is still low. Use of hydrogen for transportation applications requires materials that not only store hydrogen at high density but also operate reversibly at relatively low temperatures and pressures. Among many materials for hydrogen storage, complex hydrides of light metals containing borohydride anions have high hydrogen capacity and, thus, have been studied extensively. However, the thermodynamic and kinetic properties of the borohydrides limit their ability to cycle hydrogen at low temperatures. An example is LiBH4. Although LiBH4 has a high enthalpy of formation, ΔHf=−194.2 kJ/mol, the formation of LiBH4 from LiH+B or Li+B still requires elevated temperatures and pressures indicating a significant activation energy barrier. The reason for the high energy barrier has been suggested to be the general chemical inertness of boron, which may be due to the strong bonds in elemental boron (ΔHB(s)→B(g)=560 kJ/mol).
Considerable effort has been devoted to the development of materials that can lead to the reversible formation of borohydride anions at lower temperatures. However, even with the use of MgB2, a reduction of the temperature for reversible dehydrogenation of LiBH4 of only about 200° C. (from 600° C. to about 400° C.) has been realized in a MgB2/LiBH4 hydrogen storage system. Thus, despite this effort, the reversible temperature for borohydride-based materials is still too high, for example, for an ideal hydrogen storage system for vehicles.